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Old 08-08-2009, 07:28 AM   #1
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Improved Gas Mileage

I would like to know what people have done to improve gas mileage and engine performance on their 05 Journeys with CAT c7 350 engines?

Not sure what if anything can be done beyond proper driving skills, but I want to be sure I am not missing anything.

Thank you
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Old 08-08-2009, 12:36 PM   #2
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Hi scottma,

If you are driving 60 MPH, accelerating slowly, and anticipating stops so that the engine brake does all the stopping, then you are doing about all you can other than driving 57 MPH. What exactly is you gas mileage and how do you drive?

One thing you may consider is to have the engine exhaust brake reprogrammed to the ‘Latch’ mode. This setting makes so much more sense than the default setting and feels more natural to drive.

After 90,000 miles on our CAT C7 we plan our trips on an estimated 9 MPG and usually get 9.1 or 9.2 MPG depending on wind. If there is a really bad head wind, the mileage could be as low as 8.5 MPG. I set the cruse on either 58 or 60 MPH.

I am unaware of any mods. for the C7 to improve efficiency. Diesel engines are almost inherently as efficient as they can get. More HP is a different story but is almost always at the expense of MPG.
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Old 08-08-2009, 03:50 PM   #3
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I always accelerate slowly. I drive with the engine break on. I cruise at 65 MPH , tow a 5200 lb Land Rover and get about 7.2 MPG. I would happily lower my speed to 60 MPH if it will improve my milelage by 2mpg. How does the latching mode work ?
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Old 08-08-2009, 04:28 PM   #4
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I also want to follow this thread very closely. We have a 2005 Journey 36G with the same Cat C7 350hp engine and we have gotten an average 7.5 mpg for all our trips. We live at an elevation of one mile above sea level. We get about 8.5 mpg going down to sea level and about 6.5 mpg going home. Toad is a 3000 lb. CRV.

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Old 08-08-2009, 05:13 PM   #5
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A technique I learned when I drive my Prius might help. I'll have to try it out next trip.

I think it's called "pulse driving" where you intentionally let speed accumulate or even accelerate down hill (to whatever number you feel is safe and legal enough), then coast going up the next grade, just barely feathering the gas pedal when you start to crank down to the speed limit.

In the Prius it would increase my mileage about 3-4 mpg (from 56 to about 60 mpg) in mixed driving. In the big, blocky, unstreamlined bus I drive now, I'm not so sure.

Anyone tried this?
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Old 08-08-2009, 05:45 PM   #6
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A technique I learned when I drive my Prius might help. I'll have to try it out next trip.

I think it's called "pulse driving" where you intentionally let speed accumulate or even accelerate down hill (to whatever number you feel is safe and legal enough), then coast going up the next grade, just barely feathering the gas pedal when you start to crank down to the speed limit.

In the Prius it would increase my mileage about 3-4 mpg (from 56 to about 60 mpg) in mixed driving. In the big, blocky, unstreamlined bus I drive now, I'm not so sure.

Anyone tried this?
You bet, Rich, I do this all the time. I also try to keep my speed in the 56-58 range; doesn't always work, but I really make it point to not run over 60. Pushing that big frontal area down the boulevard does take fuel; pushing it faster takes more.
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Old 08-09-2009, 01:29 AM   #7
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Hi All,

I use Rich’s technique all the time also. The Latch mode (and a few other settings) let this happen naturally. Ken is quite right; our drag coefficient (CD) is probably on the order of .6 or higher for those aeronautically inclined members. This compares to something like a Buick Regal with a CD of probably less than .25. As speed increases, the drag goes up as to the square of the speed. So, for instance, the difference in drag going 60 verses 65 is an increase of approximately 17% or a decrease in fuel efficiency of say 20% when additional frictions are added like increased RPM and tire rotational speed.

As should be apparent, headwinds are a MPG killer. As an example, lets assume a speed of 60 MPH with a 10 MPH tailwind verses a 10 MPH headwind. The other drags will be about the same since the engine RPM and tire rotational speeds are the same. The headwind will increase drag by 96% over the tailwind. I keep telling my wife that we should never plan a trip; just see witch way the wind is blowing that day!

Hi Suncircles & Ken,

Pilots say “The net effect of the wind is to slow you down” or in our case, “The net effect of the wind is to lower MPG”. I too have gotten lousy MPG coming west on I40 into headwinds. The worst was 7.2 MPG. This is somewhat made up for by the occasional tailwind. The best we have ever had was a tank full that worked out to 10.35 MPG. Our engine computer via the Palm Pilot and CAT ‘s Pocket Tec software (no longer available) is reporting 90,914 miles on 10690 gallons of diesel for an overall average of 8.50 MPG. Bear in mind that when new from the dealer in Tucson, the computer read 6.3 MPG. It’s taken this long to get it up there but I’m quite happy with its performance now. Idle fuel is another issue as well as generator use. I use a figure of .33 gallons per hour for the generator and subtract that from the total fuel purchased to get a more accurate MPG. The engine computer also says I have used 13 gallons of fuel idling.

Scottma,

To answer your question, the ‘Latch’ mode leaves the engine brake off when you let off the throttle and it only comes on after you press the service brake (or hold down the ‘cancel’ button) for a set length of time. (.5 seconds works well) This let’s you coast down hills without the brake coming on. There is another setting involved that determines at what speed, over that set for cruse, that the engine brake will finally come on. I have mine set to 15 MPH, which leaves the brake off on all but the steepest and longest hills. Since I set 58 or 60 MPH in cruse, that means the brake will come on at 75 MPH out on a freeway.

Other operations are changed also. Of course, when the engine brake ‘switch’ is off the engine brake is off. When the switch is on and the cruise is off, the brake operates in either the ‘Coast’ or the ‘Manual’ mode. You can tell which by the way it works. With the switch on, in ‘Manual’ mode, the engine brake comes on whenever you let off the throttle. With the switch on, in ‘Coast’ mode the engine brake comes on as soon as the service brake is pressed and off when the service brake is released. When the switch is on, and Cruise is on, and ‘Latch’ mode is set, then it works as described above. This prevents needlessly slowing down when you just tap the brake to release the cruise control. Not slowing down so much means not having to accelerate to regain the speed. Ergo less fuel is used. You know, those times when you see a light change to red way up in front of you and you want to just let off the throttle and not use either the service or engine brake to save fuel and reaching to turn off the switch is more trouble than it’s worth. ‘Latch’ mode does just that. And of course, as soon as you step on the accelerator, the engine brake turns off if it was on.

Another setting to make sure is turned on is called ‘Soft Cruise’.

I think you will find that the best fuel economy will occur at that set cruise speed that just lets the transmission drop into 6th gear and stay there at least on little slopes. This assumes that that speed will be somewhere around 55 to 60 MPH. The C7 is most efficient at 1440 RPM producing enough torque (read Horse Power) to maintain speed. As RPM goes up, MPG goes down, because of friction losses in the engine. If your couch drops into 6th gear at just under 1500 RPM and 57 MPH, then you are set up to get the best MPG. Dropping your speed from 65 to 60 MPH should increase your mileage from 7.2 to close to 8.5 MPG. That Land Rover is the rest of your lost fuel efficiency.

Any CAT dealer can reset the computer with the appropriate settings. If you can find someone with a CAT Pocket Tec, then it’s very easy to read out the data and reset whatever you want including the speedometer calibration. It’s nice to have it dead on. If you are ever in Albuquerque, I have the cables for both the CAT 3126 and the CAT C7. Just look us up at Enchanted Trails RV Resort at the top of 9 mile hill west of town. Space C1, until we move to Mississippi of course.
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Old 08-09-2009, 09:10 PM   #8
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Thank you very much. I may look you up next time I am in Duke City . I have an office near Jefferson and Alameda I visit. I usually stay at the Stagecoach Stop.
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Old 08-09-2009, 09:30 PM   #9
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MrTransistor:

Thanks also for the very useful, detailed information. Our stick house is in the ABQ area and I may take you up on your offer. My '05 Journey has only 17,000 miles on it, so I am still breaking it in. 9k of it just since last Nov.

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Old 08-09-2009, 11:27 PM   #10
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Originally Posted by scottma View Post
I would like to know what people have done to improve gas mileage and engine performance on their 05 Journeys with CAT c7 350 engines?

Not sure what if anything can be done beyond proper driving skills, but I want to be sure I am not missing anything.

Thank you
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Old 08-10-2009, 05:01 PM   #11
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I agree with Mr. Transistor. Both CAT and Cummins have information on their web sites concerning fuel mileage. Both of them say that above 50mph aerodynamics is the largest effect on fuel mileage. For example, above 55 every 1mph increase in speed lowers fuel mileage 0.1mpg. As you increase your speed you increase aerodynamic drag and the horsepower needed to maintain that speed. More horsepower, more fuel. No free lunch.
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Old 08-10-2009, 10:30 PM   #12
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i just know how you wouldn't be a hazard to navigation at 50-55 on the 70 mph interstates
i applaud those who can drive that speed safely.
i just cant

we just ran a 1873.8 mile trip in long mountain climbs in new mexico and colorado
used 231.4 gallons, which is 8.09 mpg this was done at 65-66, except when coating down levita pass, this i did in 3rd with full exhaust brake
this also included 2 full days of 10 hrs of genny usage under 3/4 - full load
I had to sleep in the street at the kids house one night ( don't ask)
the few places we had to run 555 for road construction my instant mpg actually stayed in the 8.0- 8.9 range,

we also drove with severe side winds and head winds.
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Old 08-11-2009, 06:02 AM   #13
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AS noted above and from an engineering view, the biggest factor on fuel economy is speed. If you double your speed, the drag factor goes up by 4 times. Going from 60 to 65 mph, is only and 8% speed increase, but the drag is more than 17%.

You need to keep your engine in top shape, filters clean, drive easily and try to keep the engine well into the torque band. I know my Ford diesel is happiest between 1900 and 2000 rpm. start over that, with or without the trailer, the fuel economy drops. Over 2100 and it drops like a rock.

Ken
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Old 08-11-2009, 07:14 PM   #14
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Hi Robert,

I’m surprised that you don’t do the calculations on generator fuel usage and then subtract that from fuel purchased to determine MPG. Quick math says your actual mileage is possibly over 8.3 MPG. In my log, I record the Hobbs reading every time I fuel up. Then, using a standard consumption of .33 gallons per hour (there are reasons for this value), I subtract the proper amount from the fuel total to determine the actual MPG. I also record the total gallons reported by the engine to cross check the numbers.

If your actual MPG ‘is’ on the order of 8.4 or so, at a speed of 65 MPH, I think your fuel consumption would be considered quite normal.
As to driving 60 MPH on an interstate highway when the posted speed limit is 75. No problem. Just stay in the right lane and let the gas-guzzlers pass on the left doing 90. The key for me is ‘spatial awareness’. As I drive, I am always aware of the traffic ahead, behind, and beside me and what each is doing. Maybe this comes from pilot training. I never pass (it’s pretty seldom at 60) unless there are no cars any where close to me coming up from behind. I always use turn signals before, not during, the move so that drivers ahead and those far behind know what’s happening. 91,000 on this coach, 45,000 on each of the two previous coaches, and still no problems.

Now if we can only get the fuel efficiency of diesel power plants up over 80%, then we too could get 20 MPG.
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